The Hamburg data protection commissioner, already at odds with Facebook over its use of face recognition technology, reopened its proceedings against the company in August last year, telling the company to either obtain explicit consent for face recognition from users, delete the data, or face a lawsuit, Caspar said at the time.
Facebook turned off facial recognition for all European users in September last year, and said it would delete all face recognition templates for existing users in Europe.
The German commissioner stopped its proceedings against Facebook in February, when it confirmed that the company had deleted the facial recognition data gathered on German users without their consent.
Turning on facial recognition again in Germany might be illegal, Caspar said, adding that it depends on how Facebook implements it. The social network should ask for the explicit and informed consent of the user, Caspar said.
"That means that there has to be offered an opt in for users," he added.
Tag suggest is used in the U.S. in the same way it was used in Europe before it was turned off. Facial recognition software is used to calculate a unique template of a user's appearance based on facial features using variables such as the distance between the eyes, nose and ears.